The present invention relates to an optical disc drive capable of recording and/or reproducing data to/from various types of optical discs having different characteristics, such as a thickness of a protective layer and a data recording density. The present invention also relates to an objective Lens and an optical head to be employed in the above-described optical disc drive.
There are plurality of standards regarding the characteristics of the optical discs, including the thickness of the protective layer which covers a data recording surface of the optical disc and/or the data recording density. For example, the thickness of the protective layer of a CD (Compact Disc) or a CD-R (CD recordable) whose recording density is relatively low is 1.2 mm, while that of a DVD (Digital Versatile Disc) whose recording density is relatively high is 0.60 mm.
For recording and/or reproducing data to/from the DVD, since it has a relatively high data recording density, in order to make the size of a beam spot sufficiently small, a laser beam whose wavelength is in a range of approximately 635-660 nm is to be used. For the CD-R, in view of its reflection characteristics, a laser beam whose wavelength is approximately 780 nm is to be used.
The above-described optical discs may preferably be used in a single optical disc drive. In order to allow a single optical disc drive to use both the DVD and the CD-R, the disc drive is required to have at least two laser sources respectively emitting the laser beams as described above.
In the meantime, in view of downsizing of the disc drive, it is preferable that the optical system adopted in an optical head for the disc drive is as compact as possible. In particular, optical elements such as an objective lens is preferably used for both laser beams, which enables the laser sources (i.e., laser diodes) to be implemented in a single package and provided as a light source module.
If the two laser diodes are incorporated in a single package, beam emitting points of the two laser diodes are aligned in a direction perpendicular to an optical axis of the objective lens. Therefore, at least one of the beam emitting points is located off the optical axis. Further, since a single optical system is used for two different wavelengths, coma cannot be compensated for both types of optical discs whose protective layers having different thickness. Therefore, in the above-described type of optical disc drive, it is relatively difficult to balance the degree of compensation of coma and arrangement of the beam emitting points.
It is therefore an object of the invention to provide an improved optical disc drive that utilizes a light source module including at least two laser diodes emitting laser beams having different wavelengths, and a single objective optical system for both wavelengths. In the improved optical disc drive, aberration can be well suppressed with allowing recording and reproducing of data for a plurality of types of optical discs to be performed. A further object of the invention is to provide an optical head for such an optical disc drive, and an objective lens therefor.
According to an aspect of the invention, there is provided an objective lens that converges a first laser beam having a first wavelength on a first optical disc having a first protective layer, and a second laser beam having a second wavelength, which is longer than the first wavelength, on a second optical disc having a second protective layer, data recording density of the first optical disc being greater than data recording density of the second optical disc, a thickness of the first protective layer being smaller than a thickness of the second protective layer, a first region being defined on the objective lens, the first region providing a numerical aperture appropriate for converging the second laser beam on the second optical disc, coma of the objective lens in the first region being compensated better in a case where the second laser beam is converged on the second optical disc than a case where the first laser beam is converged on the first optical disc.
With this configuration, aberration can be well suppressed for both the first and second laser beams, with allowing recording and reproducing of data for a plurality of types of optical discs to be performed.
Optionally the objective lens may preferably satisfy a sine condition when the second laser beam is converged on the second optical disc.
Optionally, the objective lens may satisfy the following condition:
0 less than |SC2/SC1| less than 0.2,
where,
SC1 represents an offence SC against sine condition at the peripheral portion of the first region, taking the protective layer of the first optical disc into account, when the first laser beam is con-verged on the first optical disc,
SC2 represents an offence SC against sine condition at the peripheral portion of the first region, taking the protective layer of the second optical disc into account, when the second laser beam is converged on the second optical disc, and
the offence SC against the sine condition is defined by the formula below:
SC=nH1/(nxe2x80x2 sin Uxe2x80x2)xe2x88x92f(1xe2x88x92m)
where,
n represents a refractive index on the beam incident side medium (i.e., the air),
nxe2x80x2 represents a refractive index on the beam emerging side medium (i.e., the protective layer),
Uxe2x80x2 represents an angle of the emerging beam with respect to the optical axis,
m represents a paraxial magnification,
H1 represents a ray height on a principal plane, and
f represents a focal length.
It should be noted that, when the offence SC of the sine condition is calculated, the protective layer of the optical disc is taken into account.
Still optionally, the objective lens may consist of a single lens element, one of refraction surfaces being formed with a diffractive lens structure exhibiting spherical aberration which is changed in an undercorrected direction as the wavelength increases.
Further optionally, the diffractive lens structure includes a plurality of concentric annular zones.
In a particular case, the first wavelength is approximately 660 nm, and the second wavelength is approximately 785 nm.
According to another aspect of the invention, there is provided an optical head for an optical disc drive, which is provided with a first laser diode that emits a first laser beam having a first wavelength, a second laser diode that emits a second laser beam having a second wavelength which is longer than the first wavelength, beam emitting points of the first laser diode and the second laser diode being close to each other, an objective lens, the objective lens converging the first laser beam on a first optical disc having a first protective layer to form a beam spot, the objective lens converging the second laser beam on a second optical disc having a second protective layer to form the beam spot, data recording density of the first optical disc being greater than data recording density of the second optical disc, a thickness of the first protective layer being smaller than a thickness of the second protective layer, coma of the objective lens being compensated better in a case where the second laser beam is converged on the second optical disc than a case where the first laser beam is converged on the first optical disc, an actuator that drives the objective lens so that the beam spot traces a data track on the optical disc, a photo detecting system that receives the beam reflected by the optical disc, a beam splitting element that directs the beam reflected by the optical disc to the photo detecting system.
Optionally, the first laser diode and the second laser diode may be packaged in a single module.
Alternatively, the first laser diode, the second laser diode and the photo detecting system may be packaged in a single module.
Still optionally, a beam emitting point of the first laser diode is located closer to the optical axis of the objective lens than a beam emitting point of the second laser diode.
In a particular case, the beam emitting point of the first laser diode is located on the optical axis of the objective lens.
It should be noted that there are three possible arrangements of the beam emitting points of the laser diodes in relation to compensation for coma.
(1) The beam emitting points may be arranged such that coma is evenly compensated for both the shorter wavelength laser beam and the longer wavelength laser beam.
(2) Coma is compensated for the shorter wavelength laser beam, and the beam emitting point of the longer wavelength laser beam, for which the coma is not compensated, is located on the optical axis.
(3) Coma is compensated for the longer wavelength laser beam, and the beam emitting point of the shorter wavelength laser beam is located on the optical axis.
According to method (1), in either wave length, coma remains. According to method (2), the beam emitting point for the shorter wavelength beam is located at an off-axis position. In this case, astigmatism is relatively large and a desired or appropriate beam spot cannot be formed on the first optical disc. According to method (3), which is employed in an embodiment of the invention, the beam emitting point for the longer wavelength beam is located off the optical axis, and therefore, the objective lens exhibits astigmatism for the longer wavelength beam. Since the longer wavelength beam is utilized, the astigmatism is sufficiently small.
According to another aspect of the invention, there is provided an optical disc drive, provided with an optical head, which includes the optical head as described above, and further, a laser driver that drives the first and second laser diodes to emit the first and second laser beams, a current-to-voltage converter that converts electrical current output by the photo detecting system to a voltage signal, an operation circuit that obtains position control signal for the beam spot in accordance with the voltage signal, a controller the drives the actuator in accordance with the position control signal, a motor that rotates the optical disc, a course actuator that moves the optical head in a radial direction of the optical disc, and a signal processing circuit that reproduces data recorded on the optical disc in accordance with the voltage signal.